The present invention relates to the field of wireless communications. More particularly, the present invention relates to a time division duplex (TDD) communication system which uses dynamic link adaptation for transmissions between user equipment (UE) and a base station (BS) to adjust for changing propagation conditions.
Third generation (3G) cellular systems are able to transmit a wide range of services, from high data rate services such as video and Internet downloads, to low data rate services such as speech. Referring to FIG. 1, a plurality of user services are shown as individual data streams. These individual data streams are assigned to transport channels A, B, C, whereby the data streams are coded and multiplexed. Each transport channel A, B, C is assigned a specific coding rate and a specific transmission time interval (TTI). The coding rate determines the number of transmitted bits of the physical layer, and the TTI defines the delivery period of the block of data to be transmitted. For example, the TTI may be either 10, 20, 40 or 80 ms.
Multiple transport channels A, B, C are multiplexed together into a coded composite transport channel (CCTrCh). Since the CCTrCh is made up of a plurality of transport channels A, B, C, it may have a plurality of different coding rates and different TTIs.
For example, transport channel A may have a 20 ms TTI and transport channel B may have a 40 ms TTI. Accordingly, the formatting of transport channel A in the first 20 ms and the formatting of transport channel A in the second 20 ms can change. In contrast, since transport channel B has a 40 ms TTI, the formatting, and hence the number of bits, are the same for each 20 ms period over the 40 ms TTI duration. It is important to note that all of the transport channels A, B, C are mapped to the CCTrCh on a TTI basis, using the smallest TTI within the CCTrCh. The transmit power is ultimately determined based on transport format combination applied in the smallest TTI within the CCTrCh.
It should be noted by those of skill in the art that each individual data stream will have an associated data rate, and each physical channel will have an associated data rate. Although these data rates are related to each other, they are distinctly different data rates.
Once the smallest TTI within the CCTrCh has been established, it must be determined how many bits of data will be transmitted and which transport channels will be supported within a given TTI. This is determined by the formatting of the data.
A transport format combination (TFC) is applied to each CCTrCh based on the smallest TTI. This essentially specifies for each transport channel how much data is transmitted in a given TTI and which transport channels will coexist in the TTI.
A TFC set is the set of all of the possible TFCs. If the propagation conditions do not permit all of the possible TFCs within the TFC set to be supported by the UE, a reduced set of TFCs which are supported by the UE is created. This reduced set is called a TFC subset. TFC selection is the process used to determine which data and how much data for each transport channel A, B, C to map to the CCTrCh. A transport format combination indicator (TFCI) is an indicator of a particular TFC, and is transmitted to the receiver to inform the receiver which transport channels are active for the current frame. The receiver, based on the reception of the TFCIs, will be able to interpret which physical channels and which timeslots have been used. Accordingly, the TFCI is the vehicle which provides coordination between the transmitter and the receiver such that the receiver knows which physical transport channels have been used.
In TDD, the UE typically calculates the required transmit power based upon a signal to interference ratio (SIR) target that it receives from the base station. Knowing the TFC selected, the UE calculates the required transmission power. If the RF propagation conditions are optimal, a TFC will be selected such that the maximum number of bits are transmitted in each timeslot. However, as RF propagation conditions deteriorate and the UE calculates a required power that is higher than the maximum allowable power of the UE in order to transmit all of the desired information, a different set of TFCs, (i.e., the aforementioned TFC subset), must be selected which will be supportable by the maximum allowable power of the UE. This ultimately reduces the amount of data that the physical layer has to support, and reduces the power requirement.
In summary, the system chooses on a TTI basis which transport channels will be active and how much data will be transmitted in each one. The TFC selection process takes into account the physical transmission difficulties, (maximum allowable power being one), and reduces the physical transmission requirements for some time duration.
After the multiple transport channels A, B, C are combined into a single CCTrCh, the CCTrCh is then segmented and those segments are mapped separately onto a number of physical channels. In TDD systems, the physical channels may exist in one, or a plurality of different timeslots, and may utilize a plurality of different codes in each timeslot. Although there are as many as 16 possible codes in a timeslot in the downlink, it is more typical to have, for example, 8 codes in a particular downlink in a particular timeslot. In the uplink, there is rarely more than two codes in a particular timeslot. In any event, there are a number of physical channels defined by a plurality of codes in a plurality of timeslots. The number of physical channels can vary.
In the Universal Mobile Telecommunications System (UTMS) time division duplex (TDD) mode, the CCTrCh is mapped onto the physical channels by assigning the timeslots and the codes in consecutive order. For example, the first timeslot is selected for mapping. The first code of the first timeslot is assigned first, and then the remaining codes of the first timeslot are each assigned consecutively until the last code has been assigned. Once all of the codes from the first timeslot are assigned, the second timeslot is entered. The mapping process is repeated using each of the codes from the second timeslot consecutively until they have all been assigned.
The mapping process for a specific user equipment (UE) under UMTS is shown in the example of FIG. 2A having 12 timeslots (S1-S12), 8 codes in each timeslot (0-7), and 12 total codes (A1-A12) to be allocated/configured. Those codes and timeslots that are shown as “shaded” are considered, for purposes of illustration, not to be allocatable to the present UE, (since they may have been allocated to other UEs). The allocatable portions of timeslots S4-S7 will be assigned in consecutive order starting at timeslot S4, and codes 0-4 in each timeslot will be also assigned in consecutive order. Assuming that 12 codes will be mapped in this manner, the result is a mapping shown in FIG. 2A with code A1 being assigned first and code A12 being assigned last.
Although the prior art process shown in FIG. 2A provides one option for mapping the data from the CCTrCh onto the physical channels, there are some drawbacks with this process when transmission problems are encountered within a single timeslot, for example, when the desired transmission power exceeds the maximum allowable UE power. The process of consecutive assignment of timeslots and codes for mapping the CCTrCh onto the physical channels as set forth in the UMTS-TDD standard tends to exaggerate the problems when a transmission problem occurs. By way of illustration, due to the consecutive manner in which timeslots are allocated/configured when a transmission problem occurs, it typically occurs in one or several of the earlier timeslots. When the system detects a problem, for example, when the desired transmission power exceeds the maximum allowable UE power for a certain TTI, the system selects new TFCs such that the data requirements on all of the timeslots are reduced. Since the UMTS-TDD standard specifies that timeslots are assigned consecutively, if the transmission problem is in one of the first several timeslots, the system will still begin packing data into the earlier timeslots, where the problem is at its worst, and will leave the last timeslots relatively empty, where there are no transmission problems.
As a result, the system exacerbates the problem since data rate requirements are lowered on the timeslots where there is not a problem, and timeslots that have a problem will still be packed with data. This is an inefficient utilization of the radio resources.